In WO No. 87/00085 (Labsystems Oy) some electrically operated pipettes are described. The pipette of FIG. 1 comprises an electric motor by means of which a piston is moved back and forth in a cylinder for sucking liquid into the tip of the cylinder and dispensing it out of the tip. The pipette further comprises an adjustable upper limiter for a rack movement. The adjustable limiter is attached to a screw parallel to the direction of the movement, on which a nut knob is fitted as revolving in a hole in the body of the pipette. Thus, by turning the knob, it is possible to move the limiter in the direction of the rack movement. In this structure there is the drawback that the knob can be turned by accident.
The cited pipette further comprises at the extreme positions of the rack movement limit switches by means of which the motor is stopped. The switch may be a mechanical microswitch or, e.g., a reed tube or a Hall element. In practice, however, the operation of these switches has turned out to be inaccurate.
The cited pipette also comprises a displaceable cylinder part comprising the piston and its cylinder. When separate cylinder tips are used, they have to be removed from the cylinder e.g. by pulling with hands. It would, however, be preferable to remove the tip without touching it with hands.